XFAIL crasher for swift-5.0-branch

The previous commit causes it to regress. It doesn't crash on master and
it's a wild crash-on-invalid so I'm just going to leave it as-is.

Author: slavapestov

validation-test/compiler_crashers_fixed/28815-formprotocolrelativetype-swift-protocoldecl-swift-genericsignaturebuilder-potent.swift

@@ -6,5 +6,8 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
 // RUN: not %target-swift-frontend %s -emit-ir
+
+// XFAIL: *
+
 extension CountableRange{{}{}protocol P{typealias a:A{}func 丏
 protocol A:CountableRange